Pitch adjustment of the fan blades of a gas turbine engine may be used to optimize the efficient operation of the engine throughout its flight envelope or to provide reverse thrust on landing. Such adjustment can improve fuel economy and reduce engine noise, although precise and reliable pitch is important. Conventional pitch change mechanisms are derived from propeller installations where continual changes of pitch are required during flight. These mechanisms are mainly driven by hydraulic or electrical power supplied from a source external to the engine or generated locally to the pitch change mechanism.
Providing either hydraulic or electrical power from an external source results in extra lines through the engine and requires a means for transferring high hydraulic pressure or a large flow of electrical power from static to rotating parts. Generation of the required power locally to the pitch change mechanism on the rotating part of the engine also requires additional parts in the form of a hydraulic pump or electrical generator. All of this results in additional cost, weight and maintenance.
With a turbo-fan engine it may only be necessary to select a limited number of fan pitch settings, for example, at take-off, cruise and reverse thrust, requiring only intermittent operation of the mechanism for short periods of time in the flight cycle. Furthermore, with a turbo-fan it is sometimes necessary to inspect blades in situ and replace individual ones. This can be difficult due to the relative close positioning of the blades but the problem can be eased by varying the pitch setting. With conventional pitch change mechanisms this may require running the engine, making fan maintenance time consuming and expensive.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for changing the pitch of rotating blades. Other objects include the provision of a pitch change apparatus for rotating blades which:
1. is light weight and reliable, PA0 2. includes failsafe mechanisms in the event of power failure, PA0 3. is durable and requires minimum maintenance, PA0 4. can be operated with and/or without engine operation, PA0 5. allows pitch coarse and fine adjustment at variable rates, PA0 6. is inexpensive and of simplified construction, and PA0 7. can be operated selectively and intermittently during engine operation.